Generators 200kW No Voltage

hello,

I have stranges things on port diesel generator on a high speed ferry I am working on:

- engine is caterpillar C9

- alternator (200kW) is leroy somer LSA M 46.2L9 C6/4, 400V, 3P, 50Hz, 1500rpm, regul R448 AREP

Nearly each time we start engine, speed is OK but we get no voltage out of the generator, we stop engine and start again and every things is OK. One time, I tried to keep engine running and voltage came after 40seconds running. Sometimes We have to start and stop engine several times before getting voltage.

During normal operation, voltage is stable and diesel generator is working good.

We checked connections and took mesurements of resistance of the windings but found nothing strange.

rem: This Diesel Genetrator is started every 12hours.

rem: The diodes bridges and windings are quite dirty (dust)

We saw 3 technicians about this problem but they have no ideas...

do you have any ideas? do you know any test we can do to find the problem?

thanks again